I was writing an e-commerce system and needed a mock server to test the C client with. I'd heard of people using Perl for IPC apps, and a quick look at the man pages convinced me that it would be quicker to mock it in Perl than it would in C. One Pink camel book later (I was using Perl 4 and didn't know about Perl 5) and Bob's your mother's brother.

Curiously enough since Perl did the job the mock-server evolved into the full server and did several sterling years service before it was eventually rewritten in C by my successor.